Ball valves are used in a wide number of process control system applications to control some parameter of a process fluid (this may be a liquid, gas, slurry, etc.). While the process control system may use a control valve to ultimately control the pressure, level, pH or other desired parameter of a fluid, the control valve basically controls the rate of fluid flow.
Typically, a ball valve may include a fluid inlet and a fluid outlet separated by a ball element which, by rotating about a fixed axis and abutting to a seal assembly, controls the amount of fluid flow therethrough. During operation, the process control system, or an operator controlling the control valve manually, rotates the ball element against a surface of the seal assembly, thereby exposing a flow passage, to provide a desired fluid flow through the inlet and outlet and, therefore, the ball valve.
Ball valve components, including the ball element and assembly, are typically constructed of metal; this stands especially true when used in high pressure and/or high temperature applications. During operation of the valve, many components suffer wear due to repeated and extensive cycling of the valve, specifically the ball element and seal assembly, due to continuous frictional contact during the opening and closing of the valve. The problems resulting from the wear include, but are not limited to, diminished life span of the valve components, increased frictional forces between the ball element and the seal assembly, and undesirable leakage between the ball element and the seal assembly. Similarly, as the frictional forces relatively increase with the amount of wear the components experience, the dynamic performance and control characteristics within the valve are worsened, resulting in inefficiencies and inaccuracies in the valve.
In the past, attempts have been made to incorporate a biased main seal into the seal assembly to correct the above mentioned problems. This, however, has resulted in limiting the applications of the valve, including constraining the valve to limited bi-directional sealing capabilities. Furthermore, with the additional force and pressure created by the biased main seal against the ball element, additional wear between the ball element and the seal assembly, and specifically the main seal, is created. Additional attempts have also been made to correct the above problems, by mounting the ball element on a cam, such that the ball element during the initial stages of opening and closing the valve, withdraws from engagement with the main seal without rotation, but rather by moving away from the main seal in a direction normal to the surface of the ball element. This, however, has resulted in further complications, such as trapping debris between the ball element and the main seal. For example, when the media traveling through the valve contains fibrous material such as pulp stock or particles, the fibrous material may be trapped between the ball element and the main seal during the closing of the valve, effectively creating a leak path through the valve.
Therefore, there remains a need for an improved ball valve having a seal assembly and a ball element, that is capable of bi-directional sealing of the fluid, that is able to reduce the wear between the main seal and the ball element, that retains positive dynamic performance and control characteristics, and that prevents fibrous material or particles from being trapped between the ball element and the main seal.